An organic EL device is a spontaneous light-emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied.
Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on the organic EL device using organic materials as the constituting materials.
Tang et al. used tris(8-hydroxyquinolinol)aluminum for the light-emitting layer and a triphenyldiamine derivative for the hole transporting layer.
Advantages of the laminate structure include that the efficiency of hole injection into the light-emitting layer can be increased, that the efficiency of forming excitons which are formed by blocking and recombining electrons injected from the cathode can be increased, and that excitons formed among the light-emitting layer can be enclosed.
As the device structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light-emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light-emitting layer and an electron transporting (injecting) layer are well known.
To increase the efficiency of recombination of injected holes and electrons in the device of the laminate type, the structures of the device and the process for forming the device have been devised.
As the light-emitting material of the organic EL device, chelate complexes such as tris(8-quinolinolato)aluminum complexes, coumarine derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light-emitting materials, and development of a device exhibiting color images is expected (see, for example, Patent Document 1 and Patent Document 2).
It is recently proposed that a phosphorescent material is used in the light-emitting layer of an organic EL device other than a fluorescent material (see, for example, Non-Patent Document 1 and Non-Patent Document 2).
As described above, a great efficiency of light emission is achieved by utilizing an organic phosphorescent material excited to the singlet state and the triplet state in the light-emitting layer of an organic EL device.
It is considered that singlet excimers and triplet excimers are formed in relative amounts of 1:3 due to the difference in the multiplicity of spin when electrons and holes are recombined in an organic EL device. Therefore, it is expected that an efficiency of light emission 3 to 4 times as great as that of a device utilizing fluorescence alone can be achieved by utilizing a phosphorescent light-emitting material.
Such organic EL device has used a constitution in which layers such as an anode, a hole-transporting layer, an organic light-emitting layer, an electron-transporting layer (hole-blocking layer), an electron-injecting layer, and a cathode are sequentially laminated in such a manner that an excited state of a triplet or an exciton of the triplet does not quench. Each of a host compound and a phosphorescent compound has been used in the organic light-emitting layer (see, for example, Patent Document 3 and Patent Document 4).
In those patent documents, 4,4-N,Ndicarbazole biphenyl has been used as a host compound. However, the compound has a glass transition temperature of 110° C. or lower. Furthermore, the compound has so good symmetry that the compound is apt to crystallize. In addition, the compound has a problem in that a short circuit or a pixel defect occurs when a device is subjected to a heat test.
Furthermore, the following has been found: upon vapor deposition of the compound, crystal growth occurs at, for example, a site where foreign matter or a projection of an electrode is present, so a larger number of defects occur than the number in an initial state before the heat test starts, and the number increases with time.
Patent Document 1: JP 08-239655 A
Patent Document 2: JP 07-138561 A
Patent Document 3: U.S. Pat. No. 6,097,147
Patent Document 4: WO 01/41512
Non-Patent Document 1: D. F. O'Brien and M. A. Baldo et al., “Improved energy transfer in electrophosphorescent devices”, Applied Physics letters, Vol. 74, No. 3, pp 442 to 444, Jan. 18, 1999
Non-Patent Document 2: M. A. Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, Applied Physics Letters, Vol. 75, No. 1, pp 4 to 6, Jul. 5, 1999